The present invention relates to a novel block copolymer of styrene and ethylene and a method for the preparation thereof. More particularly, the invention relates to a block copolymer of styrene and ethylene having an outstandingly narrow molecular weight distribution of the molecules and imparted with the advantageous properties of both of two polymers including high impact strength, flexibility and selective gas permeability of polyethylenes and rigidity and transparency of polystyrenes as well as an efficient method for the preparation of an ethylene-styrene block copolymer by which the proportion between the contents of the polystyrene segments and polyethylene segments and the molecular weight thereof can be freely controlled.
A block copolymer is a copolymer composed of two kinds or more of polymeric segments of different types bonded together blockwise and has a possibility of being imparted with useful properties not obtained with either one of the homopolymerized base polymers by the compensation of the defective properties for each other. Accordingly, intensive investigations are under way in recent years to develop various kinds of novel block copolymers by combining two or more base polymers having greatly different properties. For example, attempts are being made to combine polar and nonpolar segments, crystalline and amorphous segments, rigid and flexible segments, hydrophobic and hydrophilic segments and other types of polymeric segments having oppositely different properties or to combine two types of polymeric segments having greatly differing property parameters such as the glass transition temperature, melting point and the like with an object to utilize the block copolymer as a high-impact resin as such, component of so-called polymer blends, non-vulcanization type rubber, selective gas-permeable membrane, biofunctional material, polymeric liquid crystal and the like as only several of the highlighted applications.
As is known, each of polystyrenes and polyethylenes is widely used in a variety of application fields. Each of polystyrene and polyethylene has its own advantages and disadvantages. For example, polystyrene resins are defective in their relatively low impact strength and flexibility while polyethylene resins have defects in their low transparency and rigidity. These defects in the properties naturally limit the applicability of the respective resins when either one of the resins is utilized alone.
Attempts and proposals have of course been made to copolymerize styrene and ethylene with an object to obtain a copolymer imparted with properties as a combination of the different properties of the two different polymers The copolymer of styrene and ethylene obtained by the simple copolymerization of a monomer mixture of styrene and ethylene, however, is a so-called random copolymer in which the monomeric moieties of styrene and ethylene are distributed and bonded together at random with absolutely no regularity so that the copolymer not always has a desirable property not possessed by either one of polystyrenes and polyethylenes. Accordingly, it would be a promising way that the copolymer has a blockwise structure composed of the segments of polystyrene and segments of polyethylene in order to obtain a polymeric resin having properties as a combination of the advantageous properties of a polystyrene and a polyethylene.
Unfortunately, no efficient and versatile methods are known in the prior art for the preparation of a block copolymer of styrene and ethylene. For example, U.S. Pat. No. 4,668,746 teaches a method for the preparation of a copolymer containing the moiety of ethylene in segments, in which ethylene is subjected to the living copolymerization with a complex of an alkyl lithium and N,N'-ethylene dipyrrolidine as a polymerization initiator or to the block copolymerization of ethylene and a monomer copolymerizable therewith by anionio polymerization. This method, however, is not advantageous because of the low living polymerizability of ethylene resulting in a broad distribution of the molecular weight in addition to the problem of the difficulty in the removal of the homopolymeric polyethylene contained in the product in a considerably large amount due to the low efficiency of block copolymerization. British patent No. 1,121,195 teaches a polymerization method by using an organic lithium compound and an amino complex. This method, however, is not practically applied to the preparation of a block copolymer of styrene and ethylene. Further, no block copolymer of styrene and ethylene can be obtained by the method disclosed in Japanese patent Publication No. 40-26301 for the copolymerization of styrene and ethylene by using a catalyst system composed of ethylene, titanium tetrahalide and an organic tin halide.
Recently, a method is proposed in European Polymer Journal, volume 17, page 1175 (1981), Journal of Polymer Science, Polymer Chemistry Edition, volume 22, page 3877 (1984) and Die Makromolekulare Chemie, volume 188, page 103 (1987) for the preparation of a block copolymer of styrene, butadiene and ethylene by the living polymerization of styrene and butadiene followed by the reaction of the active species with titanium tetrachloride and then introduction of ethylene into the polymerization mixture. This method, however, is questionable as a method for the preparation of a block copolymer of styrene and ethylene in the absence of any confirmatory report for the formation of such a binary block copolymer.
As to the preparation of a block copolymer containing polystyrene segments, it :s reported that anionic living polymerization of styrene is initiated using an alkyl lithium and then a diene monomer is copolymerized with the living polymer of styrene by the combined use of a basic ether compound such as diphenyl ether and anisole. No report :s available, however, on the applicability of this method to the preparation of a block copolymer of styrene and ethylene.